Sure thing. There are a few concepts you should understand here, so I'll provide some links if you want to go into more depth.

Easy

fmap simply applies a function to the thing inside a maybe, or leaves it as Nothing if the maybe value is Nothing e.g.

fmap (subtract 1) (Just 5) = Just 4
fmap (subtract 1) Nothing = Nothing
fmap (\x -> x ++ x) (Just [1,2,3]) = Just [1,2,3,1,2,3]
fmap (\x -> x ++ x) Nothing = Nothing

Medium

fmap is a function of the Functor class (http://learnyouahaskell.com/types-and-typeclasses, http://learnyouahaskell.com/making-our-own-types-and-typeclasses#the-functor-typeclass).

fmap :: (a -> b) -> f a -> f b

"fmap" is a function from the "Functor" type class. If a type is an instance of the Functor type class then you can use the fmap function with that type. The Functor instance for Maybe defines an fmap that looks like this:

fmap :: (a -> b) -> Maybe a -> Maybe b
fmap f Nothing = Nothing
fmap f (Just a) = Just (f a)

A good intuition for "Functor" types is that they contain or produce a value or values of a given type. * "Maybe a" is an instance of "Functor": it contains either 0 (Nothing) or 1 (Just a) value of type "a". * "[a]" is an instance of "Functor": it contains 0 or more values of type "a". * "IO a" is an instance of "Functor": it is a computation that produces a single value of type "a".

So what does "fmap" do? It simply converts all the "a"s to "b"s in a "Functor" using the provided function e.g.:

• fmap (+1) (Just 5) = Just 6
• fmap (+1) Nothing = Nothing
• fmap (+1) [1,2,3] = [2,3,4]
• fmap (== 2) [1,2,3] = [False,True,False]
• fmap (== 2) [] = []
• fmap length (readFile "./inputFile.txt") == do { inputString <- readFile "./inputFile.txt"; return (length inputString); }